This invention relates to a detector circuit employing Schottky diodes for detection of radio-frequency (r.f.) signals and other time-varying signals and, more particularly, to a detection circuit employing a matched pair of Schottky diodes disposed on a thermally stabilized substrate and coupled via a dual channel amplifier for rejection of a common mode of bias voltage drift in an output detected signal for freeing the detected signal from the effects of thermally induced drift in diode parameters.
Detector circuits are employed for detecting signals having time-varying waveforms. Of particular interest herein, are detectors of r.f. signals which may be received from a distant site via an antenna, or from other electronic equipment, and coupled to a detector circuit. With respect to the detection of an r.f. signal, it is noted that the detection of both modulated and unmodulated r.f. signals is of interest. The detection of an unmodulated r.f. signal, or a modulated r.f. signal wherein the amplitude varies slowly, is of particular interest in the construction of detector circuits because the presence of thermal drift in bias voltages or other circuit parameters may well mask or distort the detection of such r.f. signals, particularly relatively weak r.f. signals wherein the amplitude is commensurate with signal disturbances induced by drift.
It has been the practice to employ tunnel diodes in the construction of detectors because the tunnel diode has the important characteristic of operating with zero bias current and zero offset output voltage, while still maintaining good signal sensitivity. The zero offset voltage enables a tunnel diode detector to be employed with high accuracy in use with direct coupled (DC) amplifiers. It is further noted that the use of the tunnel diode as a detector with a DC amplifier avoids any significant output voltage drift with temperature, thereby avoiding errors associated with such drift.
The use of the tunnel diode, however, incurs performance disadvantages with respect to dynamic range. For example, the useful input power dynamic range is relatively small, typically a range of 40 decibels (dB). Also, the maximum power input level should be kept below, typically +15 dBm (relative to a milliwatt). Also, the recovery to high level, wide pulse signals, is poor. Available construction techniques with tunnel diodes does not lend itself to high reliability and, furthermore, the operating temperature range is limited to 100 degrees centigrade in the use of germanium as a construction material. These disadvantages may well outweigh the major benefit of temperature stability.
The foregoing disadvantages are avoided in the use of a Schottky r.f. detector diode. Such a diode has a wider useful dynamic range of typically +15 dBm to -44 dBm. Input power may be greater such as +20 dBm, without damage to the Schottky diode. Also, such diode has good recovery characteristics. Another advantage of the Schottky diode over the tunnel diode is the higher operating temperature range of +125 degrees C. The construction is more rugged and more reliable than that of the tunnel diode.
A major problem arises in the use of the Schottky diode for detection purposes due to the fact that the diode must be operated with a bias current in order to increase the sensitivity to a useful level. The bias current introduces an offset voltage which has a temperature drift coefficient of approximately 1 millivolt per degree Centigrade. The temperature induced voltage drift can generate an offset voltage which is large enough to severely mask the signal which is to be detected.